This portrayal of Boreth doesn't connect to what's going on there in TNG at all -- and you can't just conclude "it changed over 100 years" when we're explicitly dealing with something outside of time!
In terms of geography, jagged snow-capped peaks with underlying volcanic activity seems right in line with what we see in "Rightful Heir" (TNG)...
In terms of architecture, I didn't get the impression that this itself was supposed to exist in some physically unchanged state outside of time. On the contrary, time seems to be selectively
condensed in certain areas within its walls, as evidenced by the instant orchard and Tenavik's rapid aging. That coupled with the topography being in geologic flux as well could really wreak havoc in terms of structural stability, requiring constant remodeling and additions as lava flows engulf their facades and force retreat to higher ground. (Hey, if they can grow trees that fast, they certainly wouldn't lack for building materials!)
Besides, we know that Boreth doesn't
remain as strictly closed to all outsiders in the next century, because otherwise Worf wouldn't be allowed to make his pilgrimage there. For all we know, the order of monks he visited wasn't the same one as the Timekeepers seen here at all. Just because they've been the only game in town up to now, there's no reason they must still be later on. In fact, even in the present, by her comments about the monastery's dual purposes L'Rell seems to draw a distinction between the ordinary Followers of Kahless and the Timekeepers. Maybe some of the former get fed up with all the timey-wimey and decide to go build their own settlement further up the mountainside, or upon a neighboring slope?
(That is to say, I see no obstacle to seeing it that way, if we like.)
By contrast, if this is the origin of the Borg, that's potentially a brilliant continuity-enhancing move. That would explain the Borg's weird fixation on humans, and persistent inability to seal the deal on assimilating them. And that time travel move in First Contact was always so out-of-nowhere, but not so much if the Borg are themselves humans thrown back in time.
I'm not particularly interested in that angle, personally (although I can roll with it if things go in that direction). What I do find an intriguing idea though, as a couple other people have suggested, is that Control's mastery of nanotech might stem from information gathered about the Borg by Starfleet through the events depicted in
First Contact and "Regeneration" (ENT). Section 31 was already around back then, per "Affliction"/"Divergence" (ENT).
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MMoM